The Red Sox confirmed the move Friday morning, manager Terry Francona telling reporters it was a "very tough decision." Schoeneweis told reporters Friday that he was excited to be joining the Red Sox.
Schoeneweis and Nelson both worked a scoreless inning in Boston's 5-3 victory over the Minnesota Twins on Thursday. Nelson signed with the Red Sox this offseason and has been in camp the entire spring. Schoeneweis signed with the Sox after being released by the Brewers on March 25.
Schoeneweis was in Brewers camp as a non-roster invitee and had allowed six runs on seven hits in seven innings. He spent the 2009 season with the Arizona Diamondbacks, going 1-2 with six holds and a 7.13 ERA.
The left-hander's life took a tragic turn last May, when his wife, Gabrielle, was found dead in their home from a drug overdose, a combination of cocaine and the anesthetic lidocaine.
Schoeneweis took a three-week bereavement leave from the club and later was placed on the disabled list because of depression. He returned in September and made seven appearances with a 2.08 ERA, but his overall record in 45 appearances was 1-2 with a 7.12 ERA.
Schoeneweis has an April 15 opt-out clause in his contract, which calls for a base salary of $500,000 if he makes the club.
"I'm here to get ready to go to Boston,'' he said last week. "Any type of doubt about me and what happened with me [personally] last year and everything is kind of behind. My family is in a good spot. I'm in a good place. I feel better than I've felt in the last three or four years."
Nelson, who had a 4.02 ERA in 42 games with the Tampa Bay Rays last season, has a 3.38 ERA over 10 2/3 innings this spring, allowing 10 hits with 12 strikeouts and 4 walks.
Also on Thursday, right-handed reliever Scott Atchison was told he has made the club as well. Left-handed reliever Alan Embree was told he needs more time. The Red Sox would like him to stay in Fort Myers, Fla., pitch Saturday, then pitch for affiliates. He has an April 15 opt-out date in his conrtract.
Material from ESPNBoston.com's Mike Reiss contributed to this report.